Every replacement window has to meet a thermal standard, and there are two different numbers customers and Building Control talk about: the Window Energy Rating (WER) and the U-value. They measure related but different things, and you only need to hit one of them to comply. Here's what each means and how to spec a window that passes every time.
The short answer
For a replacement window in an existing dwelling, Approved Document L is satisfied by either a Window Energy Rating of band B or better, or a whole-window U-value of 1.4 W/m²K or lower. Any decent modern double-glazed unit meets this — but it must be evidenced.
U-value — how much heat escapes
The U-value measures how much heat passes through the window: watts per square metre per degree of temperature difference (W/m²K). Lower is better — a lower number means less heat lost. It's the direct, physics-based measure of thermal performance, and it's what the rest of the building envelope is specified in too, so it's the number engineers and Building Control tend to prefer.
Watch one detail: quote the whole-window U-value (frame and glass together, sometimes written Uw), not just the centre-of-glass figure. The glass alone always looks better than the complete window, and it's the whole-window number that counts for compliance.
Window Energy Rating — the fridge-style label
The WER is a traffic-light rating from A++ down to E, like the energy label on a fridge. It rolls three things into one grade: heat lost through the window (the U-value), heat gained from the sun through the glass (solar factor), and air leakage. Because it credits solar gain, a window can earn a good WER band partly by letting useful winter sun in, not only by insulating.
It's the customer-friendly number — "band A" is easy to sell — and it's a perfectly valid route to compliance. The rating comes from the British Fenestration Rating Council (BFRC) or an equivalent scheme.
| U-value | Window Energy Rating | |
|---|---|---|
| Measures | Heat loss only | Heat loss + solar gain + air leakage |
| Scale | W/m²K — lower is better | A++ to E — higher band is better |
| Part L pass (replacement) | ≤ 1.4 W/m²K | Band B or better |
| Best for | Technical / Building Control | Explaining it to the customer |
What Part L actually requires
Approved Document L covers conservation of fuel and power. For replacement windows in an existing home the bar is the WER band B / U-value 1.4 figure above. For new-build and extensions the target is tighter — typically a U-value around 1.2 W/m²K — because the whole dwelling is assessed against an energy target, so check the specific project requirement rather than assuming the replacement figure. Either way, modern sealed units with a low-e coating and warm-edge spacer bars hit these comfortably; triple glazing goes further still where a project demands it.
The bit installers get caught on: evidence
Meeting the standard isn't enough — you have to be able to show you met it. Under a competent-person scheme (FENSA, CERTASS) you self-certify the work, and the customer gets a certificate that matters at resale and to Building Control. Keep the product's WER certificate or U-value documentation on file so the performance claim is backed by paperwork, not just a spec sheet. That's what turns "it's an A-rated window" into a compliant, certifiable installation.
How to steer the conversation
- Customer asks "how efficient is it?": lead with the WER band — it's the label they understand.
- Building Control or an architect asks: give the whole-window U-value.
- Replacement job: band B / 1.4 W/m²K is the line to clear.
- New-build or extension: check the project's specific U-value target — it's usually tighter.
- Always: keep the certificate on file — compliance is about evidence, not just the number.
Every window we supply comes with its WER band and whole-window U-value documented, across both the uPVC and aluminium ranges — so the paperwork that satisfies Building Control ships with the job, not weeks later.
